The invention relates to an aperture device for a camera (for example, a video camera) that defines an aperture opening through which light can pass.
A conventional iris diaphragm aperture device uses several shutter blades for defining a variable aperture. FIG. 1 shows an example of a conventional iris diaphragm aperture device. As shown in FIG. 1, the conventional iris diaphragm comprises a cam disk 101, four aperture blades 102, a driving ring 103 rotatable about the cam disk 101, and a lens frame 105 for supporting the cam disk 101.
The cam disk 101 is fixed to the lens frame 105, and has an opening 107 for allowing light to pass therethrough. Four cam grooves 106 surround the opening 107. The driving ring 103 is provided between the cam ring 101 and the lens frame 105. The aperture blades 102 are pivoted about respective axes, and rotatably supported by the driving ring 103. Each aperture blade 102 has a cam follower 102a that engages a corresponding cam groove 106 formed on the cam disk 101. The driving ring 103 has a peripheral gear portion 107, which engages a drive gear (not shown) of an aperture driving mechanism. When the driving ring 103 rotates, the cam followers 102a move along the cam grooves 106 and the four blades 102 are rotated about their pivots. Consequently, the pivoted blades 102 swing, varying the position of the overlapping blades 102 and defining an aperture in the center of the iris diaphragm. Thus, the aperture opening is set, as shown in FIG. 2.
However, since the conventional iris diaphragm aperture device needs a multiplicity of shutter blades, a cam disk, and a driving disk, as well as supports for each, the number of parts is quite high. Furthermore, since the shutter blades are made of thin metal or plastic plate, operations to mount the shutter blades to the driving ring and to engage the cam followers to the cam grooves are complicated.
Still further, as shown in FIG. 3, especially in the case of point light sources, when the lens is focused to a certain point (point B in FIG. 3), then images representing other points having different distances from the lens (for example, points A, A', C and C' in FIG. 3) become indistinct, creating unsharp patches (patches a, a', c, and c'). As shown in FIG. 3, the shapes of the unsharp patches a, a', c, and c' are polygonal, being the same as that of the aperture opening (a hexagonal aperture is shown in FIG. 3). Accordingly, if a conventional aperture device having several shutter blades is used, then any unsharp patches have a polygonal shape, which can be seen as defects in the image. In order to change the share of the unsharp patches to circular, an aperture opening of circular shape is necessary. However, for a conventional iris diaphragm, circular opening would require an impracticably high number of shutter blades. If implemented, the number of parts becomes unacceptably high, and the assembling operation becomes unacceptably complicated.